AVR Instruction Set Nomenclature: Status Register (SREG ...

ST - Store Indirect From Register to data space using Index X
Instruction Set
Description:
Stores one byte indirect from a register to data space. For parts with SRAM, the data space consists of the register file, I/O
memory and internal SRAM (and external SRAM if applicable). For parts without SRAM, the data space consists of the register
file only. The EEPROM has a separate address space.
The data location is pointed to by the X (16 bits) pointer register in the register file. Memory access is limited to the current
data segment of 64K bytes. To access another data segment in devices with more than 64K bytes data space, the RAMPX
in register in the I/O area has to be changed.
The X pointer register can either be left unchanged by the operation, or it can be post-incremented or pre-decremented.
These features are especially suited for accessing arrays, tables, and stack pointer usage of the X pointer register. Note
that only the low byte of the X pointer is updated in devices with no more than 256 bytes data space. For such devices, the
high byte of the pointer is not used by this instruction and can be used for other purposes. The RAMPX register in the I/O
area is updated in parts with more than 64K bytes data space.
The result of these combinations is undefined:
ST X+, r26
ST X+, r27
ST -X, r26
ST -X, r27
Using the X pointer:
Operation: Comment:
(i) (X) ← Rr X: Unchanged
(ii) (X) ← Rr X ← X+1 X: Post incremented
(iii) X ← X - 1 (X) ← Rr X: Pre decremented
Syntax: Operands: Program Counter:
(i) ST X, Rr 0 ≤ r ≤ 31 PC ← PC + 1
(ii) ST X+, Rr 0 ≤ r ≤ 31 PC ← PC + 1
(iii) ST -X, Rr 0 ≤ r ≤ 31 PC ← PC + 1
16-bit Opcode :
(i) 1001 001r rrrr 1100
(ii) 1001 001r rrrr 1101
(iii) 1001 001r rrrr 1110
Status Register (SREG) and Boolean Formula:
I T H S V N Z C
- - - - - - - -
121